Chemical mechanical planarization (CMP) is an essential process in the manufacture of semiconductor chips today. Dielectric and metal layers used in chip fabrication must be made extremely flat and of precise thickness in order to pattern the sub-micron sized features that comprise a semiconductor device. During CMP, the combination of chemical etching and mechanical abrasion produces the required flat, precise surface for subsequent depositions. The polishing pad is usually made of polyurethane and has small pores to carry the slurry under the wafer. As a result of the polishing process, pad material and slurry residues collect in the pores, plugging them, and reducing the polish rate due to slurry starvation. When the pad becomes clogged, it becomes necessary to "condition" the pad to restore its full functionality. That is, the accumulated material must be removed before it completely clogs the pad and results in a smooth, glazed surface that does not effectively polish the semiconductor wafer. A nickel/chromium conditioning wheel with a surface of diamond abrasives embedded in a nickel/chromium setting alloy is used to condition the pad. The conditioning wheel is pressed against the polishing pad by a conditioning wheel actuator, e.g., a hydraulic/pneumatic arm, and the polishing pad and conditioning wheel are each rotated at prescribed speeds while de-ionized water or slurry is flowed to rinse away abraded material. The diamond elements remove embedded particles, slurry, and polishing byproducts from the polishing pad. The conditioning proceeds until the pad is "re-surfaced" and new pores are exposed.
As the conditioning wheel is rotated against the polishing pad, the wheel, setting alloy, and the diamonds come in contact with the chemical/mechanical slurry. Conditioners for an oxide polisher have a useable lifetime of about 15,000 wafers. On the other hand, conditioners for a tungsten metal polisher have a useable lifetime of only about 5,000 to 7,000 wafers due to the abrasive nature of the metal polishing slurry. Conventionally, a polishing pad would be conditioned with the pad being rotated at a specific rate while the conditioner is rotated at another specified rate, the process continuing for a prescribed time; these parameters thereby constituting a recipe. Especially in metal polishing pads, problems developed when, after conditioning, the polishing pad does not perform up to expectations for polishing the next set of wafers. Of course, the initial analysis was that the pad was the problem, and the obvious solution was to replace the pad. After changing the polishing pad several times after minimal conditioning, it became apparent that the problem must not be the pad but rather that the conditioning wheel is not properly conditioning the pad. To resolve this problem of prematurely changing the pad, an empirical approach was developed to determine when the conditioner needs to be replaced, thus the above stated 5000 to 7000 wafers. This is, of course, only an approximation and almost certainly leads to early or late changing of the conditioner.
While the exact problem of why the conditioner loses effectiveness is not known, it is known that the slurries used to planarize metal layers, especially tungsten, are very corrosive and abrasive. Thus, one plausible explanation for the conditioner's loss of effectiveness is that the chemicals of the slurry may attack the nickel/chromium setting alloy and, over time, may loosen the diamond crystals, causing them to fall out of the polishing surface. Of course, this reduces the effective surface area of the conditioning wheel and slows the conditioning process. Alternatively, it may be that particles from the pad, used slurry, and metal particles are collecting between the diamond crystals, and building up until the crystals are ineffective. The end result is that the present methods are imprecise in determining when a change of the conditioning wheel is required.
Accordingly, what is needed in the art is an apparatus and method for determining with some precision when a conditioning wheel has served its useful life and requires replacement.